This invention relates to systems and methods for determining position. More specifically, the present invention relates to an apparatus and method for determining the optimum source of data for position determination when GPS data and optical data may be available. The optimal source of position data is then used to determine position.
Historically, surveying has been accomplished using optical sighting methods. Optical sighting methods typically involve the determination of distance, vertical angle and horizontal angle and slope with reference to a known location at which a sighting device is operated (reference site) by sighting to a remote location which is positioned (the positioned site). Optical sighting methods provide a high degree of accuracy as long as the distance between the reference site and the staked site are short.
Recently, automated position determination systems have been used for position determination in surveying applications. One such system uses the constellation of Satellites in the Global Positioning System (GPS) operated by the U.S. Air Force. The GPS consists of a constellation of 24 orbiting satellites that transmit signals via microwave radio. These signals may be used by appropriately configured receivers to determine position.
One method for determining position uses the Coarse Acquisition (C/A) code from four or more satellites to determine position. The satellites mark their transmission digitally and the receiver compares the time it receives the time mark with its own time clock. The time delay, referred to as transit time, is typically in the range of about 70-90 milliseconds. Pseudoranges are then determined by multiplying transit time by the speed of radio transmissions (approximately 300,000,000 meters/second). Position is then determined using a geometric calculation that uses the ephemerides and calculated pseudoranges. GPS based positions are calculated using the World Geodetic System of 1984 (WGS84) coordinate system. These positions are expressed in Earth Centered Earth Fixed (ECEF) coordinates of X, Y, and Z axis. These positions are often transformed into Latitude, Longitude, and Height relative to the WGS84 ellipsoid.
Errors arise in the determined position due to timing/clock errors, intentional introduction of error by the U.S. Air Force (referred to hereinafter as xe2x80x9cselective availabilityxe2x80x9d or xe2x80x9cS/Axe2x80x9d) and errors due to atmospheric conditions. Atmospheric models can be used to partially correct for errors due to atmospheric conditions. However, because such corrections are inaccurate, they result in a determination of position that is not highly accurate.
For surveying applications a high degree of accuracy is required in determining position. Therefore, a position determination technique which provides the necessary accuracy by correcting for S/A, and correcting for atmospheric conditions is typically used. One such method is real time kinematic (RTK) position determination.
RTK systems typically include a reference GPS receiver and a roving GPS receiver. The reference GPS receiver receives signals from GPS satellites. Then, either correction data or raw observables data is transmitted to a roving GPS receiver. The roving GPS receiver also receives signals from GPS satellites. The signals received by the roving GPS receiver and the data from the reference GPS receiver are then used to determine the position of the roving GPS receiver with a high degree of accuracy. Typically carrier phase measurements are used to determine position in RTK systems. RTK systems provide a high degree of accuracy provided that the differential separation distance between the reference GPS receiver and the roving GPS receiver is within a predetermined range. However, at very short distances, optical methods are more accurate than RTK methods.
Optical systems are often undesirable for use in a particular survey due to obstructions and terrain contours that prevent direct visual observation of a remote location to be positioned. When obstructions prevent optical measurements or when the distances are so great that optical measurements do not provide the required accuracy, RTK systems are often used.
However, either an optical system or a GPS system alone is usually used to survey a particular location. This requires an advance determination as to which system is to be used each time a survey is to be taken. This process is time consuming and requires an in-depth knowledge of the capabilities and limitations of each system. Also, an in-depth knowledge of the location to be surveyed is required.
What is needed is an apparatus and method for surveying that incorporates the advantages of both optical systems and GPS systems. In addition, a method for accurately determining position is needed that uses both optical measurements and GPS measurements. Furthermore, a surveying system that is easy to use and operate is required.
The present invention provides a system that swiftly and automatically determines which type of data will provide the best survey of a particular site. The best source of position data is then used to determine the desired position.
In one embodiment of the present invention, the seamless surveying system includes a Satellite Positioning System (SATPS) unit, an optical unit and a rover unit. In one embodiment, SATPS signals from satellites of the US Global Positioning System (GPS) are received at the SATPS unit and are coupled to the rover unit. The rover unit includes a target that is adapted to be engaged by the optical unit for optically determining the position of the rover unit. The rover unit includes logic for determining the optimum source of positioning data to be used for determining position. When the seamless surveying system includes a SATPS unit and an optical unit, the rover unit determines whether optical data from the optical unit or SATPS data from the SATPS unit are to be used for determining position.
In one embodiment, the optimum source of position data is chosen based on time. That is, the first received source of position data is determined to be the optimum source of position data. The optimum source of position data is then used to determine position. Thus, when optical data is received first, optical data is used to determine position. Similarly, when SATPS data is received first, SATPS data is used to determine position. This allows for the fastest computation of position.
In another embodiment, the optimum source of position data is chosen based on the distance between the rover unit and the optical system. That is, because optical data give good results at short distances, if both optical data and SATPS data are available, and if the distance is less than a predetermined threshold (the optical threshold), optical data is used. Since SATPS data gives good results at longer distances, SATPS data is used when the distance is greater than or equal to the optical threshold.
In another embodiment, a weighting process is used to determine the optimum source of position data.
In yet another embodiment, the measurements from both the optical unit and the SATPS unit are combined to determine position.
The seamless surveying system of the present invention monitors multiple sources of position data and selects from the available sources of position data the optimum source of position data for a particular application. Therefore, there is no need for the user to determine which type of system to use as is required with prior art systems. Because the determination is automatic, there is no need for human intervention for changing from one system to another. In addition, the seamless surveying system of the present invention is easy to use since data from the optimum source of position data is automatically coupled to the rover unit and is used for accurate determination of position.
These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.